Thermoplastic elastomers do not require a vulcanization step, and can be molded in the same manner as thermoplastic resins. In recent years, therefore, such elastomers have been used in a broad range of fields such as automotive parts, household electrical appliance parts, electrical wire coverings, medical parts, miscellaneous footwear, toys, sporting goods, products for daily use and the like.
Among thermoplastic elastomers, styrene type thermoplastic elastomers in particular, as represented by polystyrene-polybutadiene-polystyrene block copolymers (SBS), polystyrene-polyisoprene-polystyrene block copolymers (SIS) and hydrogenation products of such copolymers, are widely used because of their low cost and superior resistance to hydrolysis.
In the case of hydrogenation products of block copolymers which have vinyl aromatic compound polymer blocks and conjugate diene compound polymer blocks, as represented by hydrogenation products of polystyrene-polybutadiene -polystyrene block copolymers and hydrogenation products of polystyrene-polyisoprene-polystyrene block copolymers, the insufficient heat resistance and weather resistance seen in block copolymers that have vinyl aromatic compound polymer blocks and conjugate diene compound polymer blocks is improved by hydrogenation; however, such hydrogenation products are inferior in terms of rubber elasticity at high temperatures, and show a large compression set at high temperatures.
In order to improve the above-mentioned point, a process is described in Japanese Patent Application Laid-Open No.59-6236 in which an elastomer composition is produced using the respective components of 100 parts by weight of (α-1) a hydrogenation product of a block copolymer which has two or more vinyl aromatic compound polymer blocks and one or more conjugate diene compound polymer blocks, 20 to 150 parts by weight of (α-2) a peroxide-crosslinkable olefin copolymer rubber, 0 to 50 parts by weight of (α-3) a peroxide non-crosslinking type hydrocarbon rubber, 80 to 300 parts by weight of (α-4) a non-aromatic rubber softening agent, 30 to 400 parts by weight of (α-5) a peroxide-decomposable olefin resin, and 0 to 900 parts by weight of (α-6) an inorganic filler, by first heat-treating the total amount of at least component (α-2) among the respective components (excluding component (α-1)) in the presence of an organic peroxide so that dynamic crosslinking is caused to take place, and then mixing this dynamic crosslinking product, component (α-1) and the remaining components. Furthermore, it is indicated in this Japanese Patent Application that in cases where the above-mentioned multi-stage partial crosslinking and mixing step is not used, the elastomer composition shows a great drop in mechanical strength.
Furthermore, in Japanese Patent Application Laid-Open No. 8-225713, a process for producing a thermoplastic elastomer resin composition is described which is characterized by the fact that in a process for producing a thermoplastic elastomer resin composition containing 100 parts by weight of (β-1) a block copolymer which has two or more vinyl aromatic compound polymer blocks and one or more conjugate diene compound polymer blocks, and/or a hydrogenation product of the same, 40 to 300 parts by weight of (β-2) a non-aromatic rubber softening agent, 1.0 to 100 parts by weight of (β-3) a peroxide-crosslinkable olefin resin and/or a copolymer rubber containing the same, and 10 to 150 parts by weight of (β-4) a peroxide-decomposable olefin resin and/or a copolymer containing the same, component (β-1), component (β-2), at least a portion of component (β-3) and a portion of component (β-4) are crosslinked by being subjected to a heat treatment in the presence of an organic peroxide, and this product of crosslinking is then mixed with the remainder of component (β-4) or the remainders of component (β-3) and component (β-4). Furthermore, in this Japanese Patent Application, it is indicated that the composition that is dynamically crosslinked in the presence of an organic peroxide and the component (β-4) that is mixed afterward are mutually dissolved and micro-dispersed in the composition, so that process-ability, fluidity, mechanical strength and the like of the thermoplastic elastomer resin composition that is obtained are improved.
However, in both of the processes described in the two Japanese Patent Applications above-mentioned, it is impossible to cause dynamic crosslinking of the components at one time in the final mixture proportions; it is necessary to use a process in which some of the constituent components are dynamically crosslinked, after which the remaining components are mixed. Accordingly, in both cases, the production process of the thermoplastic elastomer resin composition is extremely complicated, and the resulting composition is still insufficient in terms of rubber elasticity at high temperatures.
An object of the present invention is to provide a thermoplastic elastomer composition which is formed by dynamically crosslinking a mixture containing a hydrogenation product of a block copolymer which has vinyl aromatic compound polymer blocks and conjugate diene compound polymer blocks, a non-aromatic rubber softening agent, a peroxide-crosslinkable olefin resin and a peroxide-decomposable olefin resin in the presence of an organic peroxide, wherein this composition is a dynamically crosslinked thermoplastic elastomer composition which is superior on terms of various characteristics, and which can be obtained by an arbitrary heating and kneading step that includes a single-stage, especially a single-stage heating and kneading step, without an absolute need for a multi-stage partial crosslinking and mixing step such as that used in the prior art described in the above-mentioned patents. In particular, it is an object of the present invention to provide a process for producing a dynamically crosslinked thermoplastic elastomer composition which is superior in terms of pliability and superior in terms of rubber elasticity, especially rubber elasticity at high temperatures, which shows an extremely small compression set, and which makes it possible to produce molded articles that are superior in terms of high-temperature creep performance, mechanical strength, oil resistance, solvent resistance, heat resistance, weather resistance, surface characteristics and the like.
Furthermore, it is an object of the present invention to provide a thermoplastic elastomer composition which has the above-mentioned superior characteristics.